The interior trim appearance of an automobile has been a key area of marketing focus. Interiors are often given treatments which provide increases in perceived value as well as brand differentiation. Efforts to cut costs by removing even small interior trim details have met with significant adverse sales impact.
The current automotive interior styling trend is toward a softer, wrap-around styling vision which places special emphasis on the tactile feel of the interior, as well as the visual impression. The hard plastic substrates and surfaces of car interiors in past years are being replaced by the trend toward padded or cushioned surfaces, covered by either vinyl or textile materials. Accordingly, parts such as instrument panels, arm rests, center consoles, seating, head rests, door skins and body pillar trim have increasingly been provided with soft, padded interior trim for styling purposes, for noise abatement and for safety concerns.
The automotive interior surfaces just described typically comprise a structural substrate of aluminum, plastic or the like which is relatively rigid. A layer of padding and a trim panel may cover the interior surface of the substrate. The term "relatively rigid" as used herein refers to the fact that the substrate has greater stiffness than the trim panel covering it. Obviously some flexibility is found in components such as door panel substrates.
The surface of the trim panel which is exposed to the passenger is referred to as an A-side layer. Typical materials for the A-side layer are leather, vinyl or textile materials, including cloth or carpet. Vinyl materials often have a cotton knit backing and textiles usually have a latex backcoating applied by the fabric manufacturers to stabilize the cloth. While the materials used as A-side layers serve their cosmetic purposes well, none of them by themselves provide the soft, padded or cushioned effect desired, nor will they retain a shape. Accordingly, a layer of padding and structure must be provided between the substrate and the A-side layer.
A preferred approach for a padded trim panel is to attach the padding to the A-side material and pre-form this laminate to the contour of the substrate. Such a pre-formed, pre-padded panel can then be attached to the substrate in a single step by any method and without the need for laborious hand contouring. A trim panel of this type is described and claimed in pending U.S. patent application Ser. No. 08/797,643, entitled "Automotive Trim Panel and Method of Making Same", invented by Vincent H-H. Jones and David L. Simon, and U.S. patent application Ser. No. 08/797,646, entitled "Method of Thermoforming an Automotive Trim Panel", invented by Vincent H-H. Jones, David L. Simon and Scott M. Kloock, both filed on Jan. 31, 1997 and assigned to the present assignee. The disclosures of these applications are incorporated herein by reference.
As mentioned above A-side materials by themselves cannot be formed into a permanent non-flat contour. The A-side materials on their own lack sufficient stiffness to retain any particular contour or shape, i.e., they will fold or collapse under their own weight. The referenced applications describe an A-side plus foam laminate material which overcomes these drawbacks and provides a self-supporting, contoured, padded trim panel ready for attachment to a substrate in any desired manner. Process controls are provided which allow thermoforming these parts in quantity with precise, repeatable accuracy. Thermoforming is defined as the forming of a thermoplastic sheet by heating it and then placing it into a mold to shape it. Cooling takes place in the mold so when the part comes out it will be self-supporting, i.e., it will retain the shape or contour of the mold.
The present invention is a further refinement of the trim panel described in the above applications. One of the characteristics of a trim panel A-side layer is the feel or tactile reaction to the textural qualities of smoothness, flexibility and softness, such as might be obtained when one brushes his or her hand across the surface of the A-side layer. This characteristic is referred to as the "hand" or "touch" of the material. Obviously determination of a desirable hand depends on the preferences of individual users and is thus quite subjective. However, objective factors can be measured and selected to allow designers to create a hand that might vary from a soft touch (sometimes referred to as a loose hand) to a somewhat stiffer feel. The former might be appropriate for a luxury automobile while the latter might be preferred by owners of sports cars. The primary factor in defining a hand is the lateral spring rate of a material, i.e., the spring rate in a direction parallel to the surface. The perpendicular spring rate defines the amount of cushion a user will feel.
It has been found that the spring rates of thermoformable foams used in the making of automotive trim panels are primarily dependent on the density of the foam. Given the typical range of thicknesses of foam, a foam density of about 2.7 to about 4.0 pounds per cubic foot is required to achieve the perpendicular spring rate that produces the desired cushioned feel. But the lateral spring rate for foam of this density is too large to allow for a feeling of a soft touch or loose hand.